In general, the top face of a piston of a direct fuel injection diesel engine is formed as a flat face, but a direct fuel injection diesel engine for which the top face of a piston projects in a pentroof shape is known from Patent Publication 1 below.
When a cavity is recessed in a top face of a pentroof-shaped piston, the height of an opening of the cavity changes in the circumferential direction. Therefore, if the height of a bottom wall portion of the cavity is made uniform in the circumferential direction, the depth of a peripheral wall portion of the cavity changes in the circumferential direction, and the conditions in which air and fuel injected from a fuel injector are mixed become nonuniform in the circumferential direction, thus giving rise to the problems that the engine output decreases and harmful exhaust substances increase.
In order to solve such problems, in the arrangement described in Patent Publication 1, the height of the bottom wall portion of the cavity is changed so as to follow the change in height of the opening of the cavity, and the depth of the peripheral wall portion of the cavity is made uniform in the circumferential direction, thereby making the conditions in which fuel and air are mixed in the cavity uniform in the circumferential direction.
Patent Publication 1: Japanese Patent Application Laid-open No. 62-255520